An electrochemical double layer capacitor (EDLC) is an energy storage device consisting of two electrodes arranged in such a manner that one acts as the cathode and the other the anode, creating a structure that may be modeled as two capacitors in series. The device is characterized by a high power density whose value is dictated by the effective series resistance (ESR) of the device; the lower the ESR, the higher the power density. The capacitance of these devices is very large due to the effective surface of the activated carbon used to fabricate an EDLC and the spacing between the electrode and the molecules of the electrolyte that form the double layers. Indeed, an EDLC may be modeled as a conventional parallel plate capacitor using the effective surface area of the activated carbon for the surface and the molecular distance between the electrode and the electrolyte molecules as the spacing between the parallel plates. Capacitance is calculated by the formula C=kA/d, where k is a constant of proportionality, A is the effective surface area, and d is the spacing between the plates.
EDLC devices are typically manufactured by encasing the electrodes and separator material in a package, usually an aluminum housing, which is wetted with electrolyte and sealed. The resulting device is a single cell EDLC device. These devices are used in a wide variety of applications including but not limited to industrial power supplies, UPS (uninterrupted power supplies), electric vehicles, cell phones, and in many electronic appliances.
The current generation of EDLCs operates with a nominal voltage rating of approximately 2.7 volts, a limitation imposed by virtue of the electrolyte. This requires that for many applications, many EDLC cells are required to meet the needs of a particular application. Except for devices that work at or below 2.7 volts, one or more EDLC devices are used in series to provide a composite device that operates at a higher voltage. For a specific application, a series-parallel configuration is often needed, but can include inherent problems. For example, as the number of cells increases in a series configuration, it is necessary to address cell balancing to prevent premature failure of the composite device. This approach is frequently expensive and cumbersome. The configurations are also large and heavy due to many individual cells used to form the composite structure.